Lithographic printing presses use a so-called printing master such as a printing plate which is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
A typical printing plate precursor for computer-to-film methods comprise a hydrophilic support and an image-recording layer of a photosensitive polymer layers which include UV-sensitive diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used. Upon image-wise exposure, typically by means of a film mask in a UV contact frame, the exposed image areas become insoluble and the unexposed areas remain soluble in an aqueous alkaline developer. The plate is then processed with the developer to remove the diazonium salt or diazo resin in the unexposed areas. So the exposed areas define the image areas (printing areas) of the printing master, and such printing plate precursors are therefore called ‘negative-working’. Also positive-working materials, wherein the exposed areas define the non-printing areas, are known, e.g. plates having a novolac/naphtoquinone-diazide coating which dissolves in the developer only at exposed areas.
In addition to the above photosensitive materials, also heat-sensitive printing plate precursors are known. Such materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-) chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer or by particle coagulation of a thermoplastic polymer latex, and solubilization by the destruction of intermolecular interactions.
EP 950,514 describes a heat mode imaging element for making a positive working lithographic printing plate wherein a hydrophilic surface of the support is coated with a first layer, comprising a polymer, soluble in an aqueous alkaline solution, and a top layer which is IR-sensitive and unpenetrable for an alkaline developer. The top layer comprises a polymer that lowers the dynamic friction coefficient of the top layer to a value less than 0.37 for improving the transport to the laser drum. Such polymers may be selected from polytetrafluoroethylene or poly(alkyl)acrylates.
EP 1,101,608 describes a positive-working heat imagable precursor comprising a polymeric coating on a substrate. The polymeric coating comprises polymeric particles, selected from polyolefins and halogenated, especially fluorinated, polyolefins, for example of polyethylene or polytetrafluoroethylene. The particles are dispersed in a polymeric matrix, for example a phenolic resin. The polymeric particles are insoluble in the organic solvent of the coating and increase the physical robustness of the layer.
U.S. Pat. No. 6,238,831 describes the preparation of an offset printing plate with high printing run stability wherein the photosensitive layer contains homogeneously distributed polymeric particles. The polymer of these polymeric particles is first solubilised in the coating solution as a homogeneously solubilised phase and, during the drying process of the coating solution, polymeric particles are “in-situ” formed in the layer.
EP-A-1,157,829 describes the preparation of a lithographic printing plate which comprises a photosensitive composition wherein fine polymeric particles are dispersed in an aqueous resin composition. The polymeric particles are composed of a resin, having neutralised anionic groups and having a heat fusion property. When the photosensitive composition is irradiated by light, the fine polymeric particles are melted, fused and denaturated by the thermal energy so that only these irradiated area do not dissolve in the developer on processing.
U.S. Pat. No. 6,352,812 describes the preparation of a thermal lithographic printing plate which comprises on a hydrophilic surface of a support a composite layer structure composed of two layer coatings. The first layer is composed of aa aqueous developable polymer mixture containing a photothermal conversion material. The second layer is composed of one or more non-aqueous soluble polymers which are soluble or dispersible in a solvent which does not dissolve the first layer. The second layer may also contain polymeric particles, such as poly tetrafluoroethylene particles.
EP-A 0 832 739 describes a method for making a driographic printing plate comprising on a ink-accepting support an image-forming layer containing hydrophobic thermoplastic polymer particles and a compound capable of converting light into heat. The particles have a coagulation temperature above 35° C. and are preferably selected from polyethylene or polymethyl(meth)acrylate.
EP 950,516 describes a heat mode imaging element for making a lithographic printing plate wherein a hydrophilic surface of the support is coated with a first layer, comprising a polymer, soluble in an aqueous alkaline solution, and a top layer which is IR-sensitive and unpenetrable for an alkaline developer. The top layer comprises a compound that increases the dynamic friction coefficient of the top layer to a value between 0.40 and 0.80 for improving transport characteristics of the plate. Such compounds may be selected from a copolymer of polytetrafluoroethylene-propylene, a water insoluble inorganic compound having a three-dimensional structure with siloxane bonds, silica particles or hydrophobic ceramics.